Re: Getting time in realtime kernel driver

[Philippe Gerum <rpm@xenomai.org>]

Jan Kiszka <jan.kiszka@siemens.com> writes:

> On 12.05.26 18:20, François Legal wrote:
>> Hello,
>> 
>> I think I already asked about the same question some time ago, but I think I did not get a working answer, so here I am again.
>> 
>> On xenomai 3.2 + Ipipe (linux 5.4), I need to be able to get in a RT driver the current linux wall clock (impacted from NTP or PTP).
>> I found out I get the correct value with ktime_get_real_ts64. Was willing to use __ktime_get_real_seconds but this one is not precise enought for my application.
>> 
>> So my question is : is it safe to use ktime_get_real_ts64 from within an RT driver ioctl function, and if not, which API in xenomai gives the exact same time as this one ?
>> 
>
> Nope, it's not safe, neither under legacy I-pipe (your kernel is dead
> BTW) nor latest dovetail. Userspace access to the wallclock was hardened
> for out-of-band accesses, but in-kernel had no use case, thus was never
> considered.
>
> For userland, the easiest answer is moving to a dovetail kernel and then
> using CLOCK_REALTIME. Over I-pipe, we had to model this via a separate
> clock named CLOCK_HOST_REALTIME.
>
> Jan

This may help:

"ktime_get_real_fast_ns: - NMI safe and fast access to clock realtime."

Caveat: with this variant, timestamp is not guaranteed to be monotonic
across an update.

-- 
Philippe.